Road motor vehicles commonly have disc wheels which are held onto a hub of an axle by a plurality of wheel nuts, usually six, eight or ten in number, that are adapted to be threadably engaged to a corresponding number of studs equally spaced in a circle around the hub flange. The studs pass through holes in the wheel, and the wheel is secured to the hub by screwing the nuts tightly onto the studs to the correct torque.
However, the wheel nuts may loosen over time due to continuous or prolonged vibration of the vehicle and its front and rear axles. This problem may be particularly pronounced in heavy road motor vehicles and many other commercial road vehicles, which may have a larger number of axles, more than one wheel held onto each hub thereof, and very large and heavy wheels which make tightening of the nuts relatively difficult.
The tendency for wheel nuts to loosen over time in all road motor vehicles may be accelerated by a number of factors. These include a failure to initially tighten the nuts to the correct torque, the fitting of an incorrect wheel nut, the regular overtightening of the nuts with automated tools that cause stretching and fatigue of the studs, and the “settling” of the wheel on the hub that may occur after a wheel change which causes the force of the nut on the hub to be reduced.
Loosening of the wheel nuts can have serious adverse consequences. If a wheel nut becomes loose while the vehicle is in motion, the wheel may experience an increased radial load which, if unchecked, can cause severe damage to the wheel itself and/or to the stud, hub and possibly even the nut. In extreme cases, the wheel nut may unscrew from its stud or the wheel may sever the stud due to excessive bending loads, causing the wheel to detach from the hub with potentially disastrous results.
There have been many and varied attempts to address these problems, most of which have met with little or no success.
One approach has involved finding ways of preventing wheel nuts from becoming loose.
Apart from maintaining regular inspection and, if required, correction of wheel nut tighteners, well known self locking nut systems have been developed which rely on an interference fit between the thread on the stud and the thread on the nut to create a lock which stiffens the rotation of the nut, either by a portion of the nut thread being deformed and providing a resistance during the threaded engagement, or by locating a nylon or other deformable insert within the threaded bore of the nut and through which the stud cuts the thread against a stiffening resistance.
A problem with self locking nut systems is that they are designed to operate at maximum locking effectiveness for one tightening cycle only and, if used more than once, will have markedly reduced locking capability. Furthermore, the self locking nut cannot be tightened quickly by hand in the initial stage of its threaded engagement with the stud because the lock stiffens the rotation of the nut, with the result being that a spanner or other tool is required also at the initial stage, thereby considerably lengthening the time it takes to tighten the wheel nut and hence attach the wheel to the hub. These problems and shortcomings may be unacceptable to many users given that a wheel nut may have to be removed many times during the life of a vehicle. Replacement of worn out or damaged nuts would also become very expensive.
Another approach has involved finding ways to visually indicate in a timely manner when a wheel nut becomes loose so that appropriate intervention can occur to retighten or replace the nut before serious damage results.
It is known to provide loose wheel nut indicators in the form of one piece polymeric rings or caps which are firmly clipped around or over an individual wheel nut and which include a pointed or arrowhead portion to visually indicate any loosening of the wheel nut by its unwanted rotation.
A problem with loose wheel nut indicators of the type described above is that the arrowhead portion needs to be pointing in a predetermined start direction and any indication of wheel nut loosening must require the identification of deviation from that initial direction. In many cases, loosening of a wheel nut that may lead to problems could result from a rotation of greater than about 5°, but for rotations from about 5° to up to about 20°, identification of such deviation may be very difficult to visually detect or may be missed altogether with the use of such one piece, clip on, pointed indicators.
Furthermore, these indicators operate on individual wheel nuts, and so any loosening of one nut is independent of the behaviour of any of the other nuts. Therefore, each indicator does not have any means of preventing any other nut from loosening or of restraining any further loosening of the already loosened nut.
Australian Innovation Patent No. 2010 101 053, now ceased, discloses a loose wheel nut indicator in the form of a cap or cover having a body that can closely fit over a tightened nut so as to be rotatable with the nut, and which includes a dial or other means that is rotatable relative to the nut for visibly indicating any loosening of the nut. This indicator is complex in its construction, requiring both a body and a dial which are separately manufactured, together with a complex structure for their cooperation with the nut, and furthermore operates only on individual nuts.